1. Field of the Invention:
The invention pertains to the brazing together of metal members. More specifically, the invention pertains to the brazing of nickel to aluminum and particularly nickel tubing to an aluminum sheet or base.
2. Description of the Prior Art:
Historically aluminum has been a difficult metal to which to braze. In brazing either aluminum to aluminum or another metal to aluminum the fundamental problem encountered is the presence of an aluminum oxide coating upon the outside surface of the aluminum members. Such a coating is formed spontaneously when the surface of the newly formed aluminum member is exposed to the atmosphere as aluminum is a highly reactive metal. The aluminum oxide coating prevents the brazing metal from wetting the surface of the aluminum member thereby preventing a tight metallic bond being formed therewith. Hence, in any practical brazing process the aluminum oxide must first be removed before the brazing operation can be performed.
The most common prior method for brazing either aluminum or another metal to aluminum was with the use of a process termed flux brazing. In this process, the aluminum member was immersed in a bath of molten salt and brazing flux which removed the aluminum oxide coating so that the surface of the aluminum could be wetted with the brazing flux. In the case of brazing nickel to aluminum, the aluminum member was additionally caustically etched. Also, typically the nickel member was grit-cleaned with silicon carbide and pickled with nitric acid prior to brazing. Both aluminum and nickel members were usually preheated prior to immersion in the bath of molten salt and brazing flux.
Flux brazing of nickel to aluminum was further complicated by the requirement for a low fluoride content in the molten salt. It has been found experimentally that low fluoride content is required in order to achieve a sufficiently wide fillet needed for good heat transfer between nickel and aluminum members. Furthermore, it has been difficult to achieve strong brazed joints with good heat transfer characteristics because of the tendency of the brazed structures to entrap flux at the brazed joint depite the most vigorous efforts to control the process. Presence of the flux in the brazed joint causes large non-brazed areas in the joint which further can become a point of corrosion between the two metals. The corrosion problem is further enhanced by the inherent differences in electropotential between the two metals.
Aluminum has been successfully brazed to aluminum using a fluxless brazing process which is performed in either a vacuum or an inert atmosphere. Such a process is described in U.S. Pat. Nos. 3,321,828, 3,322,517, 3,373,482, and 3,373,483 and to C. J. Miller. In the described process, the two aluminum members to be joined are provided at the juncture therebetween the elements aluminum, magnesium, and silicon. These elements can be present as brazing sheets clad to one or the other of the members, a brazing powder mixture, or as sheet, strip, or wire inserts between the two aluminum members. The members and the brazing material are put in an evacuated oven and heated to a temperature of approximately 550.degree.C until a braze forms.